| 1. |
- Lindgren, Mikaela, 1987, et al.
(författare)
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Toward a Comprehensive Mechanistic Understanding of Hydrogen Uptake in Zirconium Alloys by Combining Atom Probe Analysis With Electronic Structure Calculations
- 2015
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Ingår i: ASTM Special Technical Publication. - 0066-0558. - 9780803175297 ; STP 1543, s. 515-539
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Konferensbidrag (refereegranskat)abstract
- The ability of a zirconium alloy to resist corrosion relies on a compromise between two opposing strategies. Minimizing the hydrogen pickup fraction (HPUF) by invoking metallic electron conduction in the barrier oxide results in rapid parabolic oxide growth. On the other hand, slow sub-parabolic barrieroxide growth, as reflected in rate limiting electron transport, may result in a high HPUF. The objective of the present study is to offer mechanistic insights as to how low concentrations of different alloying elements become decisive for the overall corrosion behavior. Combining atomistic microanalysis with first principles modeling by means of density functional theory, the speciation and redox properties of Fe and Ni towards hydrogen evolution are firstly explored.Complementary atom probe microanalysis at the metal–oxide interface provides evidence for Fe and Ni segregation to grain boundaries in Zircaloy-2 that propagates into the ZrO2 scale. Descriptors for how alloying elements in ZrO2 control electron transport as well as catalytic electron-proton recombination ingrain boundaries to form H2 are determined by means of theory. The findings are generalized by further atomistic modeling, and are thus put in the context of early reports from autoclave experiments on HPUFs of zirconium with the alloying elements Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Nb. A shunting mechanism which combines inner and outer hydrogen evolution mechanisms is proposed. Properties of the transient zirconium sub-oxide are discussed. A plausible atomistic overall understanding emerges.
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| 2. |
- Tejland, Pia, 1978, et al.
(författare)
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Oxidation Mechanism in Zircaloy- 2—The Effect of SPP Size Distribution
- 2015
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Ingår i: ASTM Special Technical Publication. - 0066-0558. - 9780803175297 ; 1543, s. 373-403
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Konferensbidrag (refereegranskat)abstract
- The metal/oxide interface region in Zircaloy-2 oxidized in autoclave was studied with transmission electron microscopy (TEM) and atom probe tomography. In addition to waviness on the micrometer scale the metal/oxide interface was found to have irregularities on a finer scale, and metal islands were found especially at metal hills (delayed parts of the oxidation front). The thickness of the sub-oxide layer varies considerably along the interface in the same sample, from 100 to virtually 0 nm. The sub-oxide composition may vary on a very fine scale (down to 5nm), and it can sometimes be a mixture of sub-oxides with different oxygen content. The metal matrix in contact with the sub-oxide is saturated with up to 32 at. % oxygen, and the oxygen diffusion profile in the metal is in approximate agreement with literature data for pure Zr. However, the diffusion length appears to be somewhat larger at interface metal hills than under valleys, probably for both geometrical and stress state reasons. Hydride precipitates, hardly visible in conventional TEM, give a good image contrast when employing high angle annular dark field imaging. A model for the oxidation process is presented, where the creep deformation of the metal close to the interface and the formation of lateral cracks in the oxide are of highest importance. The effect of second phase particle (SPP) size is suggested to be twofold: Small and numerous SPPs give a stronger metal and therefore higher stress in the oxide. Small SPPs also nucleate many more lateral cracks in the oxide, which gives a weaker oxide. Together this leads to formation of large cracks associated with transition in the oxidation rate at an earlier time than for a material with larger and fewer SPPs, and thereby a higher oxidation rate.
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| 3. |
- Topping, Matthew, et al.
(författare)
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The effect of iron on dislocation evolution in model and commercial zirconium alloys
- 2018
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Ingår i: ASTM Special Technical Publication. - 0066-0558. ; STP 1597, s. 796-822
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Konferensbidrag (refereegranskat)abstract
- Although the evolution of irradiation-induced dislocation loops has been well correlated with irradiation-induced growth phenomena, the effect of alloying elements on this evolution remains elusive, especially at low fluences. To develop a more mechanistic understanding of the role iron has on loop formation, we used state-of-the-art techniques to study a proton-irradiated Zr-0.1Fe alloy and proton- and neutron-irradiated Zircaloy-2. The two alloys were irradiated with 2-MeV protons up to 7 dpa at 350°C and Zircaloy-2 up to 14.7 × 1025n • m-2, approximately 24 dpa, in a boiling water reactor at approximately 300°C. Baseline transmission electron microscopy showed that the Zr3Fe secondary-phase particles in the binary system were larger and fewer in number than the Zr (Fe, Cr)2and Zr2(Fe, Ni) particles in Zircaloy-2. An analysis of the irradiated binary alloy revealed only limited dissolution of Ze3Fe, suggesting little dispersion of iron into the matrix, while at the same time a higher 〈a〉-loop density was observed compared with Zircaloy-2 at equivalent proton dose levels. We also found that the redistribution of iron during irradiation led to the formation of iron nanoclusters. A delay in the onset of 〈c〉-loop nucleation in proton-irradiated Zircaloy-2 compared with the binary alloy was observed. The effect of iron redistributed from secondary-phase particles because of dissolution on the density and morphology of 〈a〉 and 〈c〉 loops is described. The implication this may have on irradiation-induced growth of zirconium fuel cladding is also discussed.
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